The present invention relates to the general subject of circuits for powering discharge lamps. More particularly, the present invention relates to a ballast with adaptive end-of-lamp-life protection.
In electronic ballasts with a half-bridge type inverter and a direct-coupled output, it is common for a direct current (DC) blocking capacitor to be coupled in series with the lamp. During normal operation of the lamp, the voltage across the DC blocking capacitor (VBLOCK) is equal to approximately one-half of the DC rail voltage (VDC) that is supplied to the inverter. As the lamp approaches the end of its normal operating life, VBLOCK will tend to depart from its normal value of about VDC/2. Thus, a number of existing end-of-lamp-life protection circuits monitor VBLOCK as a reliable indicator of imminent lamp failure. A number of these circuits consider a lamp to be in a failure mode when VBLOCK departs from its normal value by more than a predetermined threshold amount.
In order to adequately protect the ballast from damage and avoid any possible overheating of the lamp sockets (the latter being a primary concern with small diameter lamps, such as T5 lamps), it is highly desirable that the predetermined threshold amount be suitably small in relation to the normal value of VBLOCK. As an example, in a ballast with VDC=450 volts, the normal value of VBLOCK is about VDC/2=225 volts. A typical protection circuit will consider the lamp to be in the failure mode if VBLOCK departs from its normal value of 225 volts by as little as 10 volts (i.e., 4%) in either direction; that is, the lamp is considered to be in the failure mode if VBLOCK either exceeds 235 volts or falls below 215 volts. In existing protection circuits, these minimum (i.e., 215 volts) and maximum (i.e., 235 volts) values are xe2x80x9cdesigned inxe2x80x9d; that is, they are specified on an a priori basis, regardless of the actual value of VBLOCK during normal operation.
The problem with setting such a tight band of detection (e.g., xc2x14%) on an a priori basis is that the tolerances of certain components in the ballast render such an approach unreliable at best. First, VBLOCK is generally monitored via a resistive voltage-divider network that is coupled in parallel with the DC blocking capacitor. The tolerances of the voltage-divider resistors are a first source of possible error. Secondly, the protection circuit itself generally includes a digital control circuit or microcontroller in which the supply voltage (VCC) can vary by as much as 5%. This introduces another possible source of detection error. Additionally, small differences in the dead-time and/or duty cycle at which the inverter switches are driven will cause VBLOCK to differ at least somewhat from its ideal normal value of VDC/2. Also, VDC itself has an associated tolerance (e.g., typically on the order of about 2% or so). Finally, each of the aforementioned sources of possible error is temperature-dependent to some extent, and may thus be aggravated by the often considerable changes in temperature that occur during operation of the ballast.
In order to avoid the detection problems arising from component tolerances, one would have to set a band of detection that is considerably less tight than in the above example. For instance, the band of detection would have to be increased to xc2x120 volts (rather than xc2x110 volts). Unfortunately, such xe2x80x9copening upxe2x80x9d of the band of detection degrades the quality of protection afforded by the protection circuit, and may not even be an option for ballasts that operate certain types of lamps.
What is needed, therefore, is a ballast with an end-of-lamp-life protection circuit that is capable of providing a tight band of detection and that is relatively insensitive to component tolerances and other sources of detection error. Such a ballast would represent a considerable advance over the prior art.